While I hadn't thought about it before, the non aggro-building of CC'ed targets makes a lot of sense.

What do you do when you want to control a CC'ed target? Have a tank whack it once or twice, then start CC'ing. When the CC break, it will happily run after the tank. Works on Moroes, Oz, Houndmaster packs in Shattered Halls, etc. Heck, we use to do it at Garr.

The threat-splitting itself is pretty apparent to anyone who used Priest kiting at Gluth.

Nitpick question for my own curiosity: What exactly flags a mob as crowd-controlled? Apart from the obvious choices like Trap/Poly/Seduce/Sap, would effects like Gouge/Blind (Disorients) or Stuns (Kidney Shot/Cheap Shot) or Fears also temporariarly stop a mob from adding threat to players?

It's a dumb question, granted, but accurate knowledge on what characterizes "crowd-controlled" and stops threat generation could prove to be useful, even if not at large.

Stuns don't count as CC in this case, fears do. Not sure about disorients.

Just a thought, range from the healed target may have to do with it.
Try experimenting with CC'd mobs at different ranges.

Are you sure about that? If you are number 1 on threat and you get a temporary deaggro such as fear or BoP the mob that was locked on you will turn to the next non-CCed player second to you in the aggro list. While doing any aggro move after the CC breaks on you (Battleshout, FFF etc) will cause it to turn back instantly just waiting for the mob to come back on it's own seems to work - but usually takes the mob longer to "realize" that you are back to be beat on.

I'm quite sure, that

#show Sunder Armor
/cast [target=mouseover, harm, nodead] Sunder Armor

did help me fairly well getting some aggro on sheeped/trapped/shackled targets while they are still cc'd without breaking it. Guess i have to test, if my macro is totally useless or not, assuming they dont generate aggro while crowdcontroled.

While the initial assumption about AoE healing threat is correct. That threat is divided by number of mobs. I'm pretty sure the threat-distribution is only shared between targets you currently have aggro on. I don't have any concrete data to support this, only anecdotal evidence.

The most obvious was from Uldaman, on the 5-6 "normal" golem groups in the room with the 4 guardians. As a warrior, if i just went in, spammed thunderclap, shouted a bit and started dishing out sunders, they would all stick to me, and never bother the healer. However, if the mage started AoE'ing, he would pull 2 or 3 of them pretty quickly, while those remaining on me, would beeline for the healer.

The story would be: I pull, take damage from 5 mobs, so health dropping merrily, mage gets aggro on 2-3 mobs who hadn't gotten sundered yet, large heal lands on me, at least 1 of the remaining mobs head for the healer. If we just single-target dps'ed them down, all of them except the dps target would stick to me.

On a side note, i don't recall pulling aggro on a mob while healing someone they weren't tanking, but it's been a while since last i've been healing much, too much tank shortage.

I'm pretty sure that it would have to be your target's list. Gaining threat with a target probably follows the same rules as whatever mechanic it is that puts you in combat with a target (e.g. healing/buffing a target puts you in combat with all the mobs your target is in combat with). This is backed up by my personal experiences, though maybe I'll test it tomorrow if no one else has by then.

1) should be the case.
You can see it on a lot of encounter (mag infernals) and old encounter like fankriss/razorgore etc.
When the mobs are in combat, healing is global threat to all mobs in combat.
Same with all buff-spells (thats why they nerfed battleshout tanking).

I'd be interested in any test result you get out of this, as like you, I am also sure that using this mouse-over sunder macro has given me agro on a cc'd mob for when it breaks.

If healing built threat on CCed mobs then it would be much much harder for tanks to pick up mobs after they were CCed. Healers would inevitably die every time a CC was broken.